Hi there! I checked with my copy of Tracy's book, and it says "I recommend swaddling at least up to 3 or 4 months, although some babies can go as long as 7 or 8 months."
My DS is 6.5 months, but we still swaddle him, because he's not a very calm baby. He starts crawling and can't soothe himself. But I'm aware that for safety reasons you should stop swaddling when your baby starts rolling over. I know that I'm doing wrong, but when my baby really wants activity at night, he easily unswaddles himself and starts crawling. But I don't recommend you to swaddle so long, as I understand now that it's a really big problem to teach him unswaddled sleeping. It's safer is to tuck in a light bedsheet from both sides between the mattress and the side panel of the cot, as children by 6 month can wake themselves by flinching. I got this advice from Gina Ford's book.